In conventional Raman spectroscopy, a source of electromagnetic radiation, such as a laser, will be used to excite a sample under investigation with an excitation energy. After contacting the sample, the excitation energy will be scattered. Most of the light scattered by the sample will be scattered elastically; this light is at an unshifted wavelength and may be detected after leaving the specimen. However, a relatively small portion of the laser light is scattered inelastically as a result of coming into contact with the sample. This inelastically scattered light exits the specimen at shifted wavelengths which are at both higher and lower energy states than the original laser wavelength. The light shifted to longer wavelengths is called the Stokes-shifted Raman signal, and the light shifted to shorter wavelengths is called the anti-Stokes Raman signal. The amount of the shift reflects the vibrational spectrum of the sample under examination. This Raman shift spectrum may be detected and analyzed, such as through use of a spectrograph to evaluate one or more properties or characteristics of the sample under examination.
A limitation of current Raman spectroscopy systems is a relatively limited capability to interact with a sample at different depths into the sample. Systems such as confocal Raman probes have been used to provide some variability of depth of investigation. However, these systems are relatively inefficient; and in view of the relatively small amount of Raman scattering signal that is typically available, will not be well-suited to some applications.